Creepers appear in Heart of the Swarm, where they are spawned by Slivan. They detonate a few seconds after reaching their target in a manner similar to banelings, but move slowly, hence allowing Kerrigan time to move out of the way even as they begin to detonate.
Creepers are spawned by creeper hosts in Co-op Missions. Like locusts, they fly, and swoop down to their targets, but unlike locusts, they detonate on impact. They deal bonus damage to structures. Also Murvar can spawns explosive creepers if Dehaka reach level 10 in talent progression.